Method and apparatus for extending life period of furnace roofs

ABSTRACT

A method is disclosed for the positive control of mechanical stresses in a roof during &#39;&#39;&#39;&#39;cold&#39;&#39;&#39;&#39; and &#39;&#39;&#39;&#39;hot&#39;&#39;&#39;&#39; periods in the furnace operation by applying variable distributed loads directed opposite and according to the force of gravity to the roof surface and variable bending moments to the roof edges. Also disclosed is a device comprising means disposed on the furnace roof and framework and adapted for applying said variable distributed loads to the roof surface and variable bending moments to the roof edges resting on skewbacks.

United States Patet [1 1 Panferov et a1.

[ METHOD AND APPARATUS FOR EXTENDING LIFE PERIOD OF FURNACE ROOFS [76] Inventors: Viktor Mikhailovich Panferov,

prospekt Mira, 112, kv. 157; Lev Petrovich Grunin, Znamenskaya ulitsa, 62, kv. 2; Lev Mikhailovich Ilin, Leninsky prospekt, 103, korpus 169, kv. 12; Mikhail Moiseevich Privalov, B.Dekabrskaya ulitsa, 1, kv. 49, all of Moscow; Andrei Dmitrievich Filatov, Oktyabrskaya ulitsa, 15, kv. 7, Magnitogorsk; Gennady Elizarovich Ovchinnikov, ulitsa Kalinina, 3, kv. 85, Magnitogorsk; Vadim Grigorievich Antipin, ulitsa Gorkogo, 19, kv. 11, Magnitogorsk; Viktor Andreevich Lednov, ulitsa Kalinina, 23, kv. 26, Magnitogorsk; Valery Fedorovich Tjurin, Ulyanovskaya ulitsa, 46, kv. 5, Moscow; Dmitry Samuilovich Rutman, Belorechinskaya ulitsa, 17, kv. 9, Sverdlovsk; Igor Pavlovich Basias, ulitsa Gagarina, 12, kv. 80, Sverdlovsk; German Tomasovich Tile, Zavodskaya ulitsa, 32, kv. 37, Sverdlovsk, all of USSR.

[22] Filed: Dec. 26, 1973 [21] Appl. No.: 427,523

Related US. Application Data [63] Continuation of Ser. No. 292,823, Sept. 2, 1972,

[111 3,869,996 1451 Mar. 11, 1975 Primary ExaminerKenneth W. Sprague [57] ABSTRACT A method is disclosed for the positive control of mechanical stresses in a roof during cold" and hot" periods in the furnace operation by applying variable distributed loads directed opposite and according to the force of gravity to the roof surface and variable bending moments to the roof edges. Also disclosed is a device comprising means disposed on the furnace roof and framework and adapted for applying said variable distributed loads to the roof surface and variable bending moments to the roof edges resting on skewbacks.

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PATENTEU 3.869.996 sum 100F 11 METHOD AND APPARATUS FOR EXTENDING LIFE PERIOD OF FURNACE ROOFS This is a continuation of application Ser. No. 292,823, filed Sept. 2, 1972, now abandoned. su

BACKGROUND AND SUMMARY OF THE INVENTION The present invention relates to a method and means for extending the life period of furnace roofs and may provide useful, for example, in open-hearth, electricarc, glass-melting and other furnaces and kilns.

Modern furnace operating practice has shown that the term of continuous life (campaign) of a furnace is detetmined by its roof wear. The furnace roof wears mainly due to peeling away or spalling of the parts of refractory blocks (bricks) of which the roof is constructed. As to the roof wear by sweating due to chemical and structural variations of the hot surface of a refractory brickwork, it has been observed to occur to a lesser degree as compared with the spalling-off. Thus, according to present-day estimates the spalling-off zone accounts for 70-80% of the wear of open-hearth furnace roofs constructed from chromite-magneiste bricks.

In melting furnaces thehottest (inside) layer of such roofs is subject to the influence of a variable temperature field recurrent with time (from one heating period to another). Within every such time interval a thermocycle the roof is alternately cooled to a certain temperature (this period will be further referred to as the cold period in the furnace operation) to be then heated also to a certain temperature (this period will be further referred to as the hot period). Thus, during one heating period the roof of an open-hearth furnace is cooled to a temperature of 1,400C to be subsequently heated to 1,650C. The internal layer of the furnace roof is not only subjected to thermocycling but is in a stressed condition, the stresses varying with time; it may be affected by an aggressive medium (e.g., by slags entrained to the roof surface by gas flows). Under these conditions, as it has been provided by the experiments carried out by the Institute of Mechanics of the Moscow State University, the refractory layer is subject to an accelerated process of thermal fatique, i.e, after a certain number of thermocycles the internal roof layer capable of withstanding a small number of the thermocycles loses its strength after a definite number thereof under the combined effect of forces and temperature loads and is liable to cracking (at a depth of 5-30 mm) which results in spalling-off. It has been also proved at the above lnstitute that the number of thermocycles withstood by the hot refractory layer is sharply reduced when the layer is subject to tension; on the contrary the hot layer unaffected by forces can withstand a greater number of the thermocycles; the maximum number of the thermocycles will be withstood by the refractory subject to a certain compression force.

Hence, it has been established that there exists an optimum level of compression stresses for a given refractory and for a pre-set level of thermocycling and slag attack, at which level the refractory is capable of withstanding the maximum number of thermocycles without failure.

It is an object of the present invention to provide a method and means for extending the life period of roofs of melting furnaces by reducing the rate of wear of the roofs due to spalling-off.

According to these and other objects in the method of extending the life period of the roofs of melting furnaces featuring relatively cold and hot periods, conforming to this invention, mechanical stresses in the roof are positively controlled throughout these periods, for which purpose variable distributed loads directed opposite and according to the force of gravity are applied to the roof surface and variable bending moments to the roof edges.

The above method offers a reduction in the rate of roof wear by spalling-off of the particles of refractory blocks. Within the hot period the total value of the variable distributed loads applied to the roof may be equivalent to 30-60% of the roof weight being directed opposite the force of gravity, whereas the total value of the variable distributed loads acting on the roof during the cold period may amount to up to 15% of the roof weight being directed opposite the force of gravity when the internal surface of the roof is cooled by 200300C, and coincident with the last-mentioned force when the internal roof surface is cooled by 400-600C, the bending moments applied to the roof edges being directed during the hot period in such a manner that the internal roof layer will be subject to tension while in the cold period they will cause compression of said layer.

Such direction and magnitude of the variable distributed loads and bending moments applied to the roof will make it possible to approach an optimum compression stress in the hot internal arch which will be capable of withstanding a greater number of the thermocycles.

Insofar as the thermal conditions of a melting furnace roof are recurrent due to recurrence of the technological conditions of the furnace, the basic program of loading the roof with surface forces and moments has been chosen for an average thermocycle during the furnace operation, i.e., use is made of an autonomous load control system. To take account of deviations of the thermal conditions of the roof from a pre-set value an additional control follow-up system is utilized.

In the course of the furnace operation measurements are made of actual sagging, temperature and thickness of the furnace roof, the distributed loads and bending moments applied to the roof being corrected for the deviations of the values obtained from those specified.

This will afford the possibility of controlling the value of loads exerted on the roof in case the furnace thermal conditions should deviate from the pre-set or average.

For the accomplishment of the above method, in a furnace with a roof which is assembled of refractory blocks fastened by dowels in a transverse direction and by longitudinal angle bars in a longitudinal direction and is suspended from cross bars of a furnace framework set up at span boundaries and resting on cooled skewbacks fastened to a vertical portion of the framework, provision is made for a device which, according to the invention, is fitted with a means arranged on the furnace roof and framework and adapted for applying variable distributed loads directed opposite and according to the force of gravity onto the roof surface and variable bending moments to the roof edges resting on the skewbacks.

The said means for applying the distributed loads directed opposite to the gravity force comprises transverse beams articulated with the longitudinal roof angle bars, the midportion of each beam mounting a movable block and every cross bar carrying at least one fixed block located along the same longitudinal axis with the movable blocks. Each longitudinal row of movable and fixed blocks is tied by a rope rigidly fixed with one end in the middle section of the furnace framework, while its other end is coupled through a dynamometer spring with a source of force mounted on the furnace framework. For applying the distributed loads directed according to the force of gravity between the roof and each cross bar in parallel with the latter are installed two arches whose one ends are articulated with the vertical part of the framework whereas their other ends are connected to their drive means adapted to traverse them in the plane of the furnace cross-section. Arranged between the roof and the arches, radially to the roof, are springs put on rods, one end of each of the rods being passed through the arches and fastened to the latter so as to be axially movable from the center of the furnace, the other ends thereof being articulated with the longitudinal roof angle bars. For applying the bending moments to the roof edges each skewback rests through flexible members against two vertical arms hinged to the vertical part of the framework, each vertical arm being articulated with a horizontal arm hingeed to the cross bar at the point of its attachment to the furnace framework and operated through a dynamometer with the aid of a rope running from the drive means.

With the above embodiment the roof can be subdivided into a number of sectors within which it ensures a similar application (to the roof) of concentrated forces featuring different values in different sectors. In one direction the sector can be defined by the span boundaries, i.e, by the spacing between the crossbars and in another by the number of the transverse beams carrying the movable blocks. The above design secures also a stepwise application of the bending moment to the roof edge, each step being determined by the skewback length. Thus, with the sectors having certain dimensions the above arrangement allows for specified load distribution over the roof surface throughout one thermocycle.

Moreover, for applying additional variable distributed loads directed opposite the gravity force, free ends of the rods in the above arrangement passing through the arches may be fitted with nuts, springs placed on the rod may be arranged between the nuts and the arches, the spring compression force being adjusted through the movable arches by the drives. This will en able the distributed loads applied to the roof surface to be varied within a wider range.

For adjusting the value of the distributed loads applied to the roof surface one, end of each dynamometer spring is fitted with load level limit switches, the number of the latter being determined by the number of specified load variations, whereas to the other end of the spring rests are fastened, interacting with the said switches as the spring is being compressed or elongated.

Thereby the switches will be connected with the control system which will send signals for actuating the source of force at certain moments of the furnace ther-. mocycle, after which the switch and the rest will disconnect the source of force automatically as soon as the pre-set load level is attained.

BRIEF DESCRIPTION OF THE DRAWINGS A fuller understanding of the nature of the invention will be had from an exemplary embodiment of a roof for an open-hearth furnace, to be taken in conjunction with accompanying drawings, in which:

FIG. 1 shows a lateral sectional view of a device, according to the invention;

FIG. 2 depicts a lateral sectional view of roof suspension elements, according to the invention;

FIG. 3 shows a right-hand section of a device according to the invention with respect to the longitudinal axis of the furnace on the charge side;

FIG. 4 shows a top view ofa left-hand part of a device according to the invention;

FIG. 5 is a section taken along the line V-V of FIG. 4.

FIG. 6 depicts one of the possible versions of attachment of a transverse beam to a roof according to the invention, in a lateral section;

FIG. 7 is a section taken along line VII-VII of FIG.

FIG. 8 is a section taken along line VlIIVIll of FIG.

FIG. 9 shows sections taken along line IX-IX and IXIX of FIG. 8;

FIG. 10 is a section taken along line X-X of FIG. 4;

FIG. 11 is a section taken along line XI-XI of FIG. 10;

FIG. 12 shows a layout of a drive for traversing the arches ofa device conforming to the invention;

FIG. 13 depicts a-side view of a dynamometer spring with a control system program device according to the invention;

FIG. 14 shows a view taken along arrow A of FIG. 13;

FIG. 15 is a sectional view of a schematic showing the arrangement of displacement pick-ups and temperature sensors according to the invention;

FIG. 16 shows the variation of temperature of an internal surface of a furnace roof;

FIG. 17 shows the variation of roof surface loads versus various production periods of a heating period; and

FIG. 18 is a block diagram ofa device control system conformingto the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present device is suitable for any existing roof of an open-hearth furnace, such as a barrel roof 1 (FIG. 1) installed on a casing by setting up one ring (arch) after another from refractory brick blocks, the bricks being separated by steel inserts (not shown in the drawing). The bricks in each arch are connected by dowels 2 (FIG. 2) passing near the upper edges of the bricks (in a transverse direction). The arches of the roof 1 also interposed by steel inserts are tied up in a longitudinal direction by a plurality of rows of longitudinal angle bars 3. The roof 1 is suspended in an unheated state from cross bars 4 of the furnace bracing. Usually the length of the longitudinal angle bars 3 is equal to that of the span between vertical beams 5 or the cross bars 4 of the furnace framework. Within the limits of each span the edge of the roof 1 rests on skewbacks 6 fastened to the vertical beams 5 of the framework.

A means for applying distributed loads directed opposite the force of gravity constitutes a system of transverse beam 7 (FIGS. 3 and 4) articulated with the angle bars 3 of the roof 1 and running throughout the entire area of the roof 1. Four rows of such beams 7, two beams in every span, pass along the entire roof 1. Each row of the angle bars 3 of the roofl is articulated with the transverse beam 7 by means of connecting rods 8. Mounted on the upper ends of the connecting rods 8 passing freely through the beam 7 are springs 9 and lock nuts 10. At the center of each beam 7 block 11 is set up. movable in a longitudinal sectional plane of the furnace. Two adjacent transverse beams 7 of the row that are located in each span are tied up with the aid of a longitudinal beam 12 secured on pivots of the movable blocks 11. The two adjacent transverse beams 7 may be not interconnected, each transverse beam 7 being in this case articulated with the cross bar 4 by means of an arm 13. The movable blocks 11 of the transverse beams 7 located in the central span of the furnace are articulated with the cross bars through the use of levers (not shown in the drawing). Along the line and in the plane of every row of the movable blocks 11 on the upper part of the cross bars 4 of the furnace framework two fixed blocks 14 are installed. In some cases the transverse beams 7 carrying the movable blocks 11 may be articulated with the midpoints of two arms 15 (FIGS. 6 and 7), either of which being coupled by the rods 8 with the longitudinal angle bars 3 of the roof 1. If that is. the case the lock nuts are not necessary.

Each longitudinal row of the movable and fixed blocks 11 and 14 is tied up by a rope 16, one end of the rope being attached in the middle portion of the furnace and the other passed to the furnace face end and connected to a source of force 17 which comprises a dynamometer spring 18 coupled with the aid of the rope with a drum ofa reducer 19 with a gear ratio i 1,400. The reducer 19 is connected through a mechanical safety device 20 with a second reducer 21 with a gear ratio 1' -20, the reducer shaft being connected through a coupling with a three-phase synchronous motor rated at O.5-l kW, 500700 rpm.

The above design permits surface load to be regulated in the roof cross-section in four independent steps during a single thermocycle.

For applying distributed loads directed according the force of gravity and additional loads directed opposite that force two pairs of movable arches (half arches) 22 are installed under or close by each cross bar 4 in parallel with the roof] (FIGS. 8 and 9). The arch 22 comprises two curved channel bars connected with each other by means ofa rigid joint. An extreme end of each movable arch 22 is articulated with the vertical beams 5 of the framework. Other ends of the arches 22 located at the crown of the roof 1 are connected to a drive 23 (FIGS. 10, ll, 12) shifting these ends in the cross-sectional plane of the furnace.

Between the roof 1 (FIG. 2) and arches 22 calibrated springs 24 are installed. The springs 24 are put on rods 25 of roof suspension, the rods being set up radially, articulated with the longitudinal angle bars 3 of the roof 1 and passed freely through apertures in the arches 22 to allow them to displace along their axes from the center of the furnace.

The upper free ends ofthe rods 25 ofthe roofsuspension carry nuts 26, calibrated springs 27 being mounted between the latter and the cross bar 4.

During a thermocycle the drives 23 will shift the end of the movable arch 22 so that the compression force 0f the springs 27 and, hence, the force directed opposite to the force of gravity will be changed.

Where the bottom of an open-hearth furnace is being repaired or when the roof 1 has a sufficiently small thickness the movable arch 22 will compress the springs 24 and, hence, the load applied will be directed according to the force of gravity.

As to the application of bending moments to the edges of the roof 1, it is effected by swinging the skewbacks 6 on which they rest. The supporting ends of each skewback 6 carry vertical arms 29 mounted on them through yieldable members 28, free ends of the arm being passed into the upper part of the furnace framework. The base of a prism 30 of the arms 29 rests in turn on brackets 31 fastened to the vertical beams 5 of the framework in such a manner as to pro vide a clearance of 10-15 mm. between the rear plane of the skewback 6 and vertical beam 5 of the framework. The clearance can be adjusted by means of bolts 32 passing through a supporting plate of the bracket 31. To allow for a free small displacement of the skewbacks 6 in a vertical plane a flat roller bearing 33 is provided between the supporting prism" 30 and arm 29.

The upper ends of the vertical arms 29 are connected with a rope 35 through two-link horizontal arms 34. One end of the rope 35 is coupled to a source of force 36 and another with a tension spring 37. Set up between the rope 35 and the source of force 36 are straingauge dynamometers 38 adapted to remotely proportion the bending moments.

Remote automatic or semi-automatic control of both the value and direction of the distributed loads and bending moments applied to the roof surface is accomplished, as pointed out hereinbefore, with the aid of the dynamometer springs 18 arranged between the reducers 19 and drive ropes 16. The springs 18 are secured between plates 42 and 39 (FIGS. 13, 14) and are connected with the ropes 16. On one end of the plates 39 of each spring 18 are rigidly fixed the ends of two pairs of guide bars 41 located along the axis of the spring 18 and passed through guide apertures in another plate 42. On the second plate 42 are also rigidly fixed two other pairs of guide bars 43 sliding in the guides of the opposite plate 39. Thus on each side of the plates 39 and 42 there are two pairs of bars 41 and 43 moving in opposite directions when the spring 18 is tensioned.

The rigidity of the springs 18 is calculated so that at pre-set loads their elongation will be incommensurably greater than the corresponding displacement of the roof 1. The guide bars 41 may carry limit switches 44 and the other guide bars 43 travelling in the opposite direction stops 45 or some other telemetering mem bers ensuring accordingly either stepwise or continuous automatic application of the distributed (with respect to the roof surface) loads and bending moments according to a predetermined program.

To maintain more accurately the preset stressed conditions of the roof I, particularly when the thermal conditions on the furnace deviate from the specified values, roof surface displacement pick-ups 46 (FlG. l5), temperature sensors 47 and residual roofthickness gauges are arranged on the skin and inserted in the roof 1. As to the displacement pick-ups 46 use many he made of conventional rheostat or inductive transmitters adopted for operation under high-temperature and dust-laden conditions. The displacement pick-ups 46 are attached at the level of the cross bars 4 of the furnace framework. They are coupled to the surface of the roof 1 by means of rods 48. To reduce errors occurring under the conditions of a nonstationary temperature field the rods 48 are made from materials with low linear expansion coefficient, their lower ends being produced from special refractories and the upper ones from invar.

The temperature sensors 47 are thermocouples: Pt/Pt-Rh for internal layers of the roof and chrome!- alumel for external roof layers:

The above device functions in a the following manner.

In different period of the thermocycle heat, application of surface forces featuring a certain magnitude and direction to the furnace roof and of bending moments to the roof skewbacks is accomplished in accordance with a pre-set program to approximate the kind and value of the stresses set up in the internal hot layer of the roof both in the cold and hot periods of operation to an optimum compression stress under which the layer can withstand a greater number of thermocycles. Substantially the possibility of controlling external loads acting on the roof in accordance with a pre-set program is based on the fact that repetitive heats lead to repetitive heat fluxes affecting the roof of an openhearth furnace, i.e., the internal layer of the roof is subject to thermocycling, the roof being loaded by a force (stress) reiterated from one thermocycle to another and varying throughout one thermocycle per se, and also to saturation with slags at a certain speed. On the basis of analysis and processing ofa large number of experimental graphs showing temperature variations of the internal hot surface of the open-hearth furnace roof, it is possible to make general approximation of these graphs, such as shown in FIG. 16. An ascending branch in the graph (curves k and g) illustrates a rise in the roof temperature which starts after pouring iron. This gradual rise in the roof temperature (from l,400 to l,650C) lasts throughout the subsequent operations of melting, refining and poling to complete readiness of the steel being melted. At the beginning of tapping the roof temperature commences to decrease descending branch of the graph in FIG. 16). A gradual drop in the roof temperature continues during subsequent operations of patching and charging (l400C). As the charge is being heated, the roof temperature undergoes only small changes (a lower horizontal section of the graph in FIG. 16). The graph in FIG. 16 takes account of the major features in the temperature variations of the internal surface of the roof, but disregards small alterations in the roof temperature related to, e.g., every reversal and some other minor operations. Proceeding from this approximation of the temperature conditions of the hot roof surface and heat exchange between the external (cold) roof surface and the furnace environments (with due regard to other constant external loads, strain and thermal strength properties of refractories, subject to partial changes under the effect of slags and swelling of the hot layer saturated with ferrous slags for a split roof with metal inserts) a threeposition system of additional loads and bending moments for a furnace roof has been calculated by using the method developed by the Institute of Mechanics of the Moscow State University and tested under actual operating conditions. In this case one system of additional loads and bending moments corresponds to the cold period of the heat and another to the hot pcriod. The cold period includes a time interval during which the following technological operations are carried out the tapping of steel, patching, charging, heating up and iron pouring, while the hot period of the thermocycle refers to iron pouring, melting, poling and refining.

During the cold period the surface loads amount to up to 15% of the roof weight being directed opposite the force of gravity, and for residual roof thickness of less than 200 mm they are equal to up to 15% of the roof weight and directed according to the gravity force.

During the hot period the surface loads are equivalent in value to 30-60% of the roof weight being directed opposite the force of gravity, transition time in passing over from one load system to another mounting to 20 min.

The surface loads are distributed over the roof cross section not uniformly, the load acting on two extreme sectors adjacent to the framework being 1.3 times greater than that on the middle sectors. The programmed bending moments are assumed to be changeable within small limits, i.e., constant for both the cold and hot periods.

The third system of additional loads and bending moments corresponds to a thermocycle not pertaining to the main cycle presented in FIG. 17. It is employed within the time interval when the roof is being cooled to l,l0Ol0OOC and subsequent heating to a temperature of l,500l550C when the bottom is repaired (the first repairs being intended for 40-50 heats). For this period the surface loads exerted at a minimum temperature are equal to about 5% of the roof weight being directed accordantly to the force of gravity, while the bending moments are increased by about 1.2-1.5 times, producing an additional compression of the internal layer. The control period is selected in accordance with the assumed bottom repair period.

Along with the three-position system of additional loads and binding moments a five-position system has been calculated and performed, which makes it possible to take into account alteration of the stressed roof condition even during charging and at the beginning of iron pouring. A qualitative picture of variations of the surface load in accordance with the technological periods of the melting process is given in FIG. 17.

To accomplish a threeor five-position system of additional loads and bending moments an autonomous (semi-automatic) control system has been practised. A signal for passing over from one working load system to another is sent by an operator (steel-maker) whereafter the control system will function automatically. A block diagram of the control system is given in FIG. 18.

The control system is started by the operator (steelmaker) from a control panel 49 located at the furnace control station.

The control panel is fitted with three buttons: a push button which on being depressed provides the loading of all the eight electric motors 50 of the surface load system, a push button of a release system and a third, emergency button to be used on special occasions. There is also a crank handle for setting the control system to bottom repairs conditions which correspond to a minimum load exerted by the surface forces. Besides, provision is made for three control buttons per each of the four electric motors of the source of force 36 (FIGS. 3 and 4) required for controlling the system of establishing additional instantaneous loads. Each of the four sources of force 36 of the system can be energized and deenergized by the operator (steel maker) in conformity with the readings of strain-gauge dynamometers 38 mounted on each source of force 36. The readings of the strain-gauge dynamometers 38 are automatically recorded on a multiple point apparatus. At regular intervals the loads applied are subject to alterations depending on the residual thickness of the roof 1, Le, at a certain deviation of the actual roof thickness from the calculated value the above deviations being as sumed to be equal to 60 mm. and the load values being adjusted for by an appropriate transfer of limit switches 44 (FIGS. 13, 14) on dynamometer springs 37 of the source of force 36. Measurements of the roof thickness are taken at nine points or positions on the roof, i.e., at three points per each arch in three roof spans.

Actually the residual thickness of the roof 1 is measured at these points by temperature sensors 47 (FIG. 15) set up at a distance of 50-60 mm and l mm from the hot surface of the roof, or measurement of the residual roof thickness can be effected either manually by a specially designed probe (not shown in the drawing) through openings at the same roof points or by thickness gauge pick-ups.

Along with the program described hereinbefore the stressed condition of the roof may be controlled by another program which takes into account minute changes in the rooftemperature throughout each cycle. It is known that upon cooling of the roof 1 (after the head has been tapped) the roof temperature is slightly increased (by 50l00" C), this taking place from a certain moment when the charge is being heated. Then on completion of the charge heating operation at the first moment after iron pouring the roof temperature drops again to the minimum level (l.4501,500C). When the heat is being melted and refined the temperature rises again to the upper limit (-l,650C).

The program of loading the roof with the loads distributed over its surface with due regard for the above increases and drops in the roof temperature in the middle of the cycle is shown in FIG. 17.

As can be seen in FIG. 17, at the beginning of the tapping operation a signal is sent (point a) and the load is reduced to the lower limit (until the limit switch 44 is cut out point 12). On completion of all the operations and until charging is finished at the first stage of the charge heating process a signal is produced again (point e) and the load is increased to a level corresponding to the rise in the roof temperature by some 50-l00C (until the limit switch 44 is cut out point a). When the iron is being poured the temperature of the roof is again reduced. Next on the arrival ofa signal (point e) the load is again decreased to the lower limit (until switching off the limit switch 44 pointj). On cutting out the electric motors 50 and the sources of force 17 by corresponding limit switches 44 a master switch (timer) (not shown in the drawing) is energized automatically to complete the rest of the control program. The master switch effects a requisite time delay 10-15 min.) corresponding to a time interval when the rooftemperature on being reduced to a minimum practically does not vary. Following that the roof temperature starts rising, the master switch actuates the electric motors 50 of the sources of force 17 and the distributed loads directed opposite the force of gravity increase to the upper limit (until the limit switches 44 are cut out point 11). Insofar as the rise of the temperature occurs at a lower rate than its decrease (the rate of loading being constant) the master switch ensures a stepwise loading of the roof (loading 5 min, interval 5 min.).

In repairing the bottom a corresponding signal is sent (point b), the load value is reduced to a pre-set level (until the limit switch 44 is cut out point n). After the bottom has been repaired an appropriate signal is produced (point p) and the loads are increased to the previous level (point 0).

With the present control circuit any load levels can be provided with the aid of the sources of force 17 and in accordance with the particular features of the operating conditions of the open-hearth furnace.

At the beginning of the campaign the roof 1 is constructed (assembled) in conformity with the production process adopted for the open-hearth furnaces from chromite-magnesite refractory blocks by a template procedure. Bricks are 380 or 460 mm high. In wedging up each arch, arch thrust shall be checked with a special instrument. The template radius (of the internal surface of the roof) is equal to 4,800-5,500mm The roof height at a point above the still level amounts to 3000 3200 mm. for furnaces in a ZOO-t. capacities, 3300-3600mm. for 400-t. furnaces, 3,800-3,900 mm. for 600-t. furnaces and 4,2004,300 mm. for doublebath furnaces.

On being assembled on the template in the idle period of the furnace, the roof] (FIGS. 1,8, 9) is hinged by rods 25 to its design members connecting rods 8 of suspended arches 22 and to transverse beams 7. With the aid of the sources of force 17 and 36 (FIGS. 3, 4, 5) ropes l6 and 35 are gradually tightened to avoid free play in suspension members of the roof a and in swivel elements of skewbacks 6 which may occur on applying the bending moments. The template being removed the roof 1 is kept in the arches due to the thrust and by the suspension rods 8. Next the furnace is heated according to a preset schedule. During heating the roof 1 rises by 20-40 mm., the ropes l6 and 35 being therefor re-tightened to eliminate free play which may occur in the system for creating surface and instantaneous loads. After the temperature of the roof 1 has reached the lower limit of the basic thermal cycle (l,400l,450C) steel-making operations are initiated: patching, charging and heating of the charge, Then, on tapping the iron, additional heating of the roof 1 is started. At this moment the furnace operator (steel-maker) cuts in the sources of force 17 and 36 to increase the load (by depressing the button loading"), the ropes 16 and 35 are tightened by means of drums of reducers 21 and 51 ofthe sources of force 17 and 36 whereas dynamometer springs 18 are elongated. The ropes 16 are tightened and, resting on fixed blocks 14 tend to shift the movable blocks 11 from the roof 1. The movable blocks 11 transmit the force directed opposite the force of gravity to corresponding points on the roof 1 through the transverse beams 7 and connecting rods 8. The magnitude of the loads distributed across the longitudinal zones of the roof 1 rises automatically to a pre-set value (30-60% of the weight of the corresponding roof section). Similarly, a drive 23 acting through arches 22 and rods 25 changes the loads distributed across the transverse lines and directed opposite the gravity force. Simultaneously the sources of force 36 tighten the ropes 35 and through dynamometers 38 rotate horizontal arms 34 connected thereto about their stationary axes. The arms 34 in their turn transmit the forces to the upper ends of vertical arms 29 and to the skewbacks 6 associated therewith so that the bending moment is directed to secure compression of the internal layer of the roof 1 until the preset stresses are produced therein. The value of the bending moments is controlled with the aid of the dynamometers 38 fitted with resistance transducers (not shown in the drawing) and a multiple point automatic bridgetype recorder disposed on the furnace control panel.

Thus, simultaneously with the roof 1 being heated to a maximum temperature (1,650C), i.e., on completion of the refining process when the heat is ready, the maximum pre-set values (30-60% of the roof weight) of the loads are attained the loads being distributed over the surface of the roof 1 and directed against the force of gravity, along with the pre-set values of the bending moments directed so as to ensure compression of the roof 1. Further, from the beginning of the tapping period the roof 1 commences to cool down. The furnace operator (steel-maker) at the furnace control panel sends a signal (by pushing the button release) actuating the sources of force 17 and 36 and the drives 23 of the suspended arches 22 (FIGS. 10, 11 and 12). The diminishing of the loads distributed over the surface of the roof 1 and variations of the bending moments applied to its edges occur in a reverse order. As the ropes 16 get loose on the drums of the sources of force 17 and drives 23 the dynamometer springs 18 are compressed until the limit switches 44 disconnect the circuits of the corresponding electric motors 50. A new level of the distributed loads determined by the dynamometer springs 18 (-20% of the roof weight) will be set on the surface of the roofl through the ropes l6, movable and fixed blocks 11 and 14, beams 7 and rods 8 along the longitudinal lines on one side and through the arches 22 and rods 25 on the other side of the roof. At the same time the sources of force 36 will slacken the ropes 35 and horizontal arms 34 linked with them will rotate about their fixed axes under the influence of the springs 37. The arms 34 in their turn will transmit the forces to the upper ends of the vertical arms 29 and the skewbacks 6 coupled therewith so that the bending moment will be directed to provide tension of the internal layer of the roof 1 until the stresses in the layer reach a pre-set value. The sources of force 36 are deenergized by the operator (steel-maker) in accordance with the readings ofa control instrument (not shown in the drawing) on the furnace control panel.

Hence, as the temperature of the roof 1 is reduced to minimum (l,450C), i.e. at the moment tapping is completed, the loads distributed over the surface of the roof 1 and directed opposite the force of gravity and the bending moments directed so as to provide tension in the internal layer of the roof will attain the pre-set minimum values (10-20% of the weight of the roof).

Further on the furnace roof equipped with the device described hereinabove adapted for positive loading of the roof with distributed loads and bending moments applied to the roof edges through the skewbacks, with a system for controlling said loads and a set of measuring gauges will be referred to as an optimum-stressed roof. Experience with the optimum-stresses on the open-hearth furnaces proved its high efficiency. It provides more than 1.5-fold increase in the life period of the open-hearth furnaces.

The optimum-stresses roof can be practised not only on barrel or cylindrical but also on spherical roofs of steel-melting electric-arc furnaces.

What is claimed is:

l. A method of extending the life of roofs on furnaces having relatively cold and hot periods in the course of a melting cycle, comprising: positively controlling stresses developing in said roof by simultaneously applying variable distributed vertical loads to the surface of said roof and variable bending moments to the edges of said roof and varying the loads and the moments at least twice in the course of the melting cycle. 2. The method of extending the life of roofs on furnaces as claimed in claim 1, wherein the variable distributed loads applied to the surface of said roof have the following total value: during said hot period amounting to 30-60% of the weight of said roof and being directed opposite to the force of gravity; during said cold period with the roof surface cooled by 200-300C being equal to up to 15% of the weight of said roof and being directed opposite to the force of gravity; and when the surface of said roof is cooled by 400-600C the loads being directed according to the force of gravity; and bending moments applied to the edges of said roof being directed in said hot period to provide tension of the surface of said roof and being directed in said cold period to ensure compression of the surface of said roof.

3. In a furnace comprising: a framework having an upper part and a vertical portion; cross bars fastened to the upper part of said framework; skewbacks secured to the vertical portion of said framework; a roof composed of brick blocks, suspended from said cross bars and resting on said skewbacks; dowels fastening said roof in a transverse direction; and longitudinal angle bars fastening said roof in a longitudinal direction; a device for extending the life of the roof comprising: means disposed on said roof and framework for applying variable distributed vertical loads to the surface of said roof.

4. In a furnace comprising: a framework having an upper part and a vertical portion; cross bars fastened to the upper part of said framework; skewbacks secured to the vertical portion of said framework; a roof composed of brick blocks, suspended from said cross bars and resting on said skewbacks; dowels fastening said roof in a transverse direction; and longitudinal angle bars fastening said roofin a longitudinal direction; a device for extending the life of the roof comprising: means for applying distributed vertical loads directed opposite to the force of gravity, the means being disposed on said furnace roof and framework and comprising: transverse beams articulated with said longitudinal angle bars; a movable block mounted in the middle portion of each of said transverse beams; at least one fixed block disposed on each of said cross bars on a common longitudinal axis with said movable blocks; sources of force arranged on said furnace framework; dynamometer springs each of which is connected with one of said sources of force; and ropes each of which connects the longitudinal row of said movable and fixed blocks, one end of each rope rigidly fixed in the middle portion of said furnace framework and the other end of each rope coupled through said dynamometer springs with said sources of force; means for applying variable distributed loads directed according to the force of gravity to the surface of said roof comprising:

two arches mounted between said roof and each said cross bars parallel to the latter; one end of each of the arches being articulated with the vertical part of said furnace framework and the other end connected to a drive adapted for transerving them in a cross-sectional plane of said furnace; rods mounted radially to said roofone end of each of said passing through said arches and secured to them to allow their axial travel in a direction away from the furnace center while the other end is articulated with said longitudinal angle bars of said roof; and springs fitted on said rods and disposed between said arches and the roof; and means for apply ing bending moments to the edges of said roof comprising: vertical arms supporting said skewbacks and articulated with the vertical portion of said framework; yieldable members located between said skewbacks and the vertical beams; horizontal arms one of which is articulated with one of said cross bars at the point where the latter is attached to said furnace framework and hinged to one of said horizontal arms; ropes coupled with said horizontal arms; and drives connected to the ropes for setting the horizontal arms in motion through the dynamometers.

5. The furnace of claim 4, in which the means for applying variable distributed loads directed opposite to the force of gravity further comprises: nuts fitted on the free ends of said rods passing through said arches; springs mounted between said nuts and arches; and a drive for adjusting the compression of the additional springs to apply a surface load to the roof opposite to the force of gravity.

6. The furnace of claim 4, in which each of said dynamometer spring comprises load limit switches arranged at one end of the dynamometer spring and whose number is determined by the number of load variations in the course of a melting cycle; and rests secured at the other end of the spring and interacting with said switches as said spring is being stretched or compressed. 

1. A method of extending the life of roofs on furnaces having relatively cold and hot periods in the course of a melting cycle, comprising: positively controlling stresses developing in said roof by simultaneously applying variable distributed vertical loads to the surface of said roof and variable bending moments to the edges of said roof and varying the loads and the moments at least twice in the course of the melting cycle.
 1. A method of extending the life of roofs on furnaces having relatively cold and hot periods in the course of a melting cycle, comprising: positively controlling stresses developing in said roof by simultaneously applying variable distributed vertical loads to the surface of said roof and variable bending moments to the edges of said roof and varying the loads and the moments at least twice in the course of the melting cycle.
 2. The method of extending the life of roofs on furnaces as claimed in claim 1, wherein the variable distributed loads applied to the surface of said roof have the following total value: during said hot period amounting to 30-60% of the weight of said roof and being directed opposite to the force of gravity; during said cold period with the roof surface cooled by 200*-300*C being equal to up to 15% of the weight of said roof and being directed opposite to the force of gravity; and when the surface of said roof is cooled by 400*-600*C the loads being directed according to the force of gravity; and bending moments applied to the edges of said roof being directed in said hot period to provide tension of the surface of said roof and being directed in said cold period to ensure compression of the surface of said roof.
 3. In a furnace comprising: a framework having an upper part and a vertical portion; cross bars fastened to the upper part of said framework; skewbacks secured to the vertical portion of said framework; a roof composed of brick blocks, suspended from said cross bars and resting on said skewbacks; dowels fastening said roof in a transverse direction; and longitudinal angle bars fastening said roof in a longitudinal direction; a device for extending the life of the roof comprising: means disposed on said roof and framework for applying variable distributed vertical loads to the surface of said roof.
 4. In a furnace comprising: a framework having an upper part and a vertical portion; cross bars fastened to the upper part of said framework; skewbacks secured to the vertical portion of said framework; a roof composed of brick blocks, suspended from said cross bars and resting on said skewbacks; dowels fastening said roof in a transverse direction; and longitudinal angle bars fastening said roof in a longitudinal direction; a device for extending the life of the roof comprising: means for applying distributed vertical loads directed opposite to the force of gravity, the means being disposed on said furnace roof and framework and comprising: transverse beams articulated with said longitudinal angle bars; a movable block mounted in the middle portion of each of said transverse beams; at least one fixed block disposed on each of said cross bars on a common longitudinal axis with said movable blocks; sources of force arranged on said furnace framework; dynamometer springs each of which is connected with one of said sources of force; and ropes each of which connects the longitudinal row of said movable and fixed blocks, one end of each rope rigidly fixed in the middle portion of said furnace framework and the other end of each rope coupled through said dynamometer springs with said sources of force; means for applying variable distributed loads directed according to the force of gravity to the surface of said roof comprising: two arches mounted between said roof and each said cross bars parallel to the latter; one end of each of the arches being articulated with the vertical part of said furnace framework and the other end connected to a drive adapted for transerving them in a cross-sectional plane of said furnace; rods mounted radially to said roof, one end of each of said passing through said arches and secured to them to allow their axial travel in a direction away from the furnace center while the other end is articulated with said longitudinal angle bars of said roof; and springs fitted on said rods and disposed between said arches and the roof; and means for applying bending moments to the edges of said roof comprising: vertical arms supporting said skewbacks and articulated with the vertical portion of said framework; yieldable members located between said skewbacks and the vertical beams; horizontal arms one of which is articulated with one of said cross bars at the point where the latter is attached to said furnace framework and hinged to one of said horizontal arms; ropes coupled with said horizontal arms; and drives connected to the ropes for setting the horizontal arms in motion through the dynamometers.
 5. The furnace of claim 4, in which the means for applying variable distributed loads directed opposite to the force of gravity further comprises: nuts fitted on the free ends of said rods passing through said arches; springs mounted between said nuts and arches; and a drive for adjusting the compression of the additional springs to apply a surface load to the roof opposite to the force of gravity. 